1. Field of the Invention
This invention relates to a matrix-type display and particularly to a coupled two part display, one part providing light emitting rows and the other part providing light modulating or switching columns.
2. Prior Art
There has been a long felt need for flat panel displays to replace the cathode ray tube which has dominated the display field for many years. New plasma structures, improvements in electroluminescence and liquid crystal display technology along with the concurrent improvement in high voltage and thin film transistor technology have led to small, emissive and reflective flat displays which are sold commercially
In early flat-panel displays, e.g., electroluminscent displays, each picture element (pixel) was individually driven such that N.times.M drivers and interconnects were required for a display made of N rows and M colummns and having N.times.M pixels. This type of array is only practical economically and mechanically in an array with few elements, as in difital clock displays or the like. Matrix addressed displays then evolved which allowed addressing by only N.times.M interconnections and drivers. Since the cost and reliability of the display is often determined by the requirements of the electronics driving the display, this represented a great saving over the old type flat panel display, and such matrix addressed displays became more practical with the advent of surface mounted and thin film ICs.
Matrix addressing achieves these economies in drivers and interconnections by having many pixels share the same lines. Clearly those pixels are not independent of one another. Drive signals applied to a row and column to turn on the selected pixel at the intersection may also excite the unselected pixels attached to those lines. This means the display element must exhibit a threshold in its electro-optic response: row and column signals can be set below the threshold individually so they cause no response, while their coincidence will exceed the threshold and turn the element on. However, the drive on selected pixels can not be increased arbitrarily without bringing unselected pixels above threshold. Electro-optic phenomena which lack a discrete threshold have been excluded from display applications more complex than clocks.
One example of a prior art flat panel display employs a ferroelectric (FE) switching element which has a discrete threshold in combination with an electroluminescent (EL) panel. Here, the EL material was viewed directly and the FE element determined which matrix addressed EL pixel was activated.
Another example of a flat panel display utilizes twisted neumatic liquid crystals as the display media. Generally, these displays were reflective or transmissive in nature, requiring an external light source as the source of light. More recently, ferroelectric liquid crystal displays having response times several orders of magnitude faster than the twisted nematics as well as higher contrast ratios and wider viewing angles have been reported (K. Iwasa-Journal of Electrical Engineering, pp. 33-37, September 1986).
In all of the above devices, the rows and columns are integrated into a single or unitary structure. Often thin film, e.g., amorphous silicon film, transistors are formed directly on the structure for controlling the pixels. Such unitary structures often lead to higher manufacturing costs in that the manufacturing yield is determined by the operability of the entire complex panel, rather than of simple, interchangeable parts.
Another common element of the prior art devices is that the rows and columns of the matrix display are formed of the same type of active medium, that is, a liquid crystal layer which acts as a shutter or an electroluminescent layer which gives off light at the desired pixel.